Custom Search
|
|
|
||
 1.2.3.2 Frequency Control. Electrical frequency is directly proportional to the rpm of the
rotor which is driven by the prime mover. Because of this relationship, prime movers are
controlled by governors that respond to variation in speed or frequency. The governor is
connected to the throttle control mechanism to regulate speed, accomplishing frequency control
automatically.
1.2.4 Parallel Operation of Generators. Large power plants normally have more than one
generator in operation at the same time. When generators are to be paralleled, it is necessary to
synchronize the units before closing the paralleling circuit breaker. This means that the
generators must be brought to approximately the same speed, the same phase rotation and
position, and the same voltage. Proper synchronization is accomplished with the aid of a
synchroscope, an instrument which indicates the difference in phase position and in frequency of
two sources. Paralleling of generators is accomplished either manually or automatically with one
incoming unit at a time.
1.2.5 DC Generation. The requirement for direct current power is limited largely to special
loads; for example, electrochemical processes, railway electrification, cranes, automotive
equipment, and elevators. Direct current power may be generated directly as such, but is more
commonly obtained by conversion or rectification of AC power near the load.
1.3 ALTERNATING CURRENT POWER TRANSMISSION SYSTEM. The transmission
system is the bulk power transfer system between the power generation station and the
distribution center from which power is carried to customer delivery points. The transmission
system includes step-up and step-down transformers at the generating and distribution stations,
respectively. The transmission system is usually part of the electric utility's network. Power
transmission systems may include subtransmission stages to supply intermediate voltage
levels. Subtransmission stages are used to enable a more practical or economical transition
between transmission and distribution systems.
1.3.1 Transmission Voltage. Usually, generated power is transformed in a substation, located
at the generating station, to 46 kV or more for transmission. Standard nominal transmission
system voltages are: 69 kV, 115 kV, 138 kV, 161 kV and 230 kV. Some transmission voltages,
however, may be at 23 kV to 69 kV, levels normally categorized as primary distribution system
voltages. There are also a few transmission networks operating in the extra-high-voltage class
(345 kV to 765 kV).
1.3.2 Transmission Lines. Transmission lines supply distribution substations equipped with
transformers which step the high voltages down to lower levels. The transmission of large
quantities of power over long distances is more economical at higher voltages. Power
transmission at high voltage can be accomplished with lower currents which lower the I 2 R
(Power) losses and reduce the voltage drop. The consequent use of smaller conductors
1-3
|
 
|
|
|
||
